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16                                              INTRODUCTION.
taining' phosphorus, and for the same reason, but in very much less measure, sulphur can be eliminated.
After the charge of pig-iron and scrap is melted, iron ore is added as fast as necessary to oxidize the excess of carbon, and when the metal has reached the desired composition it is tapped into the ladle, the additions of manganese being made in the same manner as in the acid furnace.
The principles underlying the reactions in a basic furnace may briefly and incompletely be stated as follows:
(1)   Silicon oxidizes readily at a high heat under almost all conditions.    Its oxide is sand (Si02), which acts as an acid, by which is meant that it will combine if it has a chance with one of the bases or earths, like lime, iron or manganese.
(2)   Phosphorus oxidizes readily, but it will not stay in the form of oxide unless the conditions are favorable.    Its oxide is phosphoric anhydride  (P205), which acts as an acid like silica; but silica when formed is stable and will stay where it is put, but the oxide of phosphorus must have something to unite with, and this something must be one of the bases or earths like lime, iron or manganese.    If oxide of phosphorus is formed, and there is no base for it to unite with, the metallic iron robs it of its oxygen, and then we have oxide of iron, while the phosphorus is left alone, dissolved in the bath.
(3)   The oxide of phosphorus requires a considerable quantity of bases to unite with.    If the quantity is limited, the phosphorus may stay for a time, but will then ]eave.    If a slag contains all the phosphorus it can hold at a certain temperature and the furnace gets hotter, some of the phosphorus will go back into the metal. If, with the same slag the carbon begins to burn faster from any cause, the phosphorus will go back into the metal on account of the reducing action being stronger.
(4)   The oxide of phosphorus does not hold on with equal force to all bases.    If it is combined with lime it is much harder to pull it back than if it is combined with iron.
(5)   Since oxide of phosphorus acts as an acid and combines with a base, it is evident that a slag which is absorbing phosphorus becomes every moment more acid, and thus becomes every moment less capable of further absorption.
(6)   It is the rule in slags that a mixture of several different acids and bases will be more active than a slag made of one acid